Dual-mode elastic-wall underwater acoustic absorber enabled by metal diaphragm for low-frequency broadband performance

To counteract sonar detection threats, the application of acoustic coatings on the surfaces of submersibles has become a primary technique for enhancing their security. Such coatings effectively absorb sound waves and improve stealth, thereby ensuring the safety of underwater operations. This paper presents a channel-type underwater sound absorber incorporating metal spacers as vibration-isolation elements, with an elastic inner wall segmented by these spacers. Simulation results demonstrate that the introduction of metal spacers enhances the independent action of the two absorption peaks, significantly improving dual-peak sound absorption performance. Energy consumption distribution analysis further reveals that the lower segment of the elastic material primarily contributes to the first absorption peak, while the upper segment mainly contributes to the second absorption peak. Through modal control and parameter matching of the two-segment elastic wall, the designed composite structure achieves sound absorption coefficients above 0.6 within the frequency range of 630-4000 Hz. Measured results from the fabricated sample show strong agreement with simulation predictions. The acoustic absorption performance of the sample was tested under atmospheric pressure and at four hydrostatic pressures (0.5 MPa, 1 MPa, 2 MPa, and 3 MPa). As the hydrostatic pressure gradually increased, the parametric gradient balance was disrupted, causing a single absorption peak to gradually shift toward lower frequencies while maintaining a near-unity absorption coefficient. Within the frequency range over which broadband absorption was maintained, the absorption coefficient remained above 0.8 over a broad frequency band.